Muffler

ABSTRACT

A muffler which can be attached to the exhaust of an engine to muffle the sound while permitting the escape of exhaust gases, including a cylindrical shell coupled to the exhaust of the engine and having numerous holes arranged in several rings. A flexible band covers each ring of holes in a sealing fit thereover, to prevent the escape of gases except when the localized pressure on the inner side of the hole is sufficient to deflect a band portion away from the hole to permit the escape of gas through the hole.

United States Patent Raudman, Jr.

1 1 MUFFLER [76] Inventor: Charles ,1. Raudman, .lr., 23350 Agramonte. Newhall. Calif. 91321 [22] Filed: Feb. 22, 1974 [21] App]. No.: 444,839

[52] U.S. Cl. 181/57; 181/47 A; 181/60; 181/65 [51] Int. Cl. FOln 1/08 [58] Field of Search 181/37, 40, 60, 65, 64 A, 181/47, 64 B, 47 A, 64, 36 A, 57, 49, 35 A; 60/312, 314

[56] References Cited UNITED STATES PATENTS 659,456 10/1900 Smith 181/60 UX 734,864 7/1903 Heaslet 1 181/60 X 1,990,249 2/1935 Picron 181/65 3,132,716 5/1964 Williams. 181/37 X 3,139,007 6/1964 Zollinger 181/64 B UX 3,379,278 4/1968 Skowron 18l/36A UX 3.434280 3/1969 Burkhart 181/40 UX 3.590345 7/1971 Murphy n 181/40 July 15, 1975 3,688,869 9/1972 Murphy 1. 181/40 3,783,590 1/1974 Allen 181/37 X FOREIGN PATENTS OR APPLICATIONS 764,347 5/1934 France 181/37 256,165 12/1927 ltaly 181/67 Primary Examiner-Richard B. Wilkinson Assistant E.\'um1ner.l0hn F. Gonzales Attorney, Agent, or FirmLindenberg, Freilich, Wasserman, Rosen & Fernandez 1 5 7] ABSTRACT A muffler which can be attached to the exhaust of an engine to muffle the sound while permitting the escape of exhaust gases, including a cylindrical shell coupled to the exhaust of the engine and having numerous holes arranged in several rings. A flexible band covers each ring of holes in a sealing fit thereover, to prevent the escape of gases except when the localized pressure on the inner side of the hole is sufficient to deflect a band portion away from the hole to permit the escape of gas through the hole.

7 Claims, 14 Drawing Figures uunnnnrm uumuuiunr "ETEWEMUL 15 ms SHEET I AT V-NTEU JUL 1 SHEET llllllllllllll fiNINE MUFFLER BACKGROUND OF THE INVENTION This invention relates to muffling devices for minimizing the passage of acoustic or shock waves in a medium while permitting the passage therethrough of the medium.

Muffling devices are utilized in a wide variety of applications to pass a gas or liquid medium while minimizing the transmittal of acoustic or shock waves that occur in the medium. One widespread application in which it has been difficult to design adequate mufflers is in the production of motorcycles, where there is limited space available for muffling devices. The diffuclty in designing muffling devices arises because designs which reduce the transmission of sound tend to also resist the escape of exhaust gases, and in the case of an engine muffler a high resistance to passage of exhaust gases results in high back pressure and consequent low engine efficiency. An engine muffling device which blocked a high proportion of the exhaust noise without creating a high back pressure would enable the construction of quiet and efficient engines, particularly for applications where limited space is available for muffling devices.

SUMMARY OF THE INVENTION In accordance with one embodiment of the present invention, a simple muffling device is provided which is highly resistant to the passage of sound or other acousticlike waves in a medium, but which provides a low resistance to the passage of the medium itself. The muffling device essentially includes a chamber with numerous holes, and valving means for sealing the holes to the flow of the medium therethrough except when the localized pressure is high enough to displace the valving means at a hole. The chamber has a cylindrical portion with holes arranged in rings thereon, while the valving means is formed by bands of flexible material which cover the holes in a sealing relationship to prevent the flow of the medium therethrough. However, when the localized pressure is high enough to deflect a portion of a band away from a hole, the band portion deflects to permit the passage of the medium.

A muffler which is especially useful for two-cycle engines includes a cynlindrical shell which is closed at one end and which is connected at the opposite end to the expansion chamber that is normally connected to the exhaust of the engine. The shell has several grooves with holes at the bottom of the grooves, and several elastic bands are disposed in the grooves to seal the holes. The engine generates pulses in the exhaust which pass through the expansion chamber and into the muffler. However, it is believed that the acoustic waves and major pressure waves resulting from the pulses of exhaust gas do not coincide. Accordingly, a peak of the pressure wave can cause deflection of an elastic band portion, to allow the escape of gases at a time when there are no large acoustic wave fronts at the hole, so that gas escapes but only a minimum of noise escapes to the environment. With respect to the acousticor shock waves generated by the engine, the muffler serves as a closed chamber which reflects the waves back towards the engine. In the case of a two-cycle engine, the shock wave reflected by the muffler serves to retain fresh charges of air and fuel in the engine, particularly at low speeds, to thereby increase the efficiency of the engine.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a muffler constructed in accordance with one embodiment of the present invention;

FIG. 2 is a sectional side view of the muffler of FIG.

FIG. 3 is a sectional enlarged view of a portion of the muffler of FIG. 2, shown at a time when the covering band seals the exhaust port;

FIG. 4 is a view similar to FIG. 3, but showing the muffler portion at a time when the band portion is unsealed from the exhaust port;

FIG. 5 is a sectional side view of a two-cycle engine shown with the muffler of FIG. 1 installed thereon;

FIG. 6 is a perspective view of a muffler constructed in accordance with another embodiment of the invention, wherein the muffler has fins that retain the elastic bands in place;

FIG. 7 is a sectional side view of a muffling device constructed in accordance with another embodiment of the invention, which is especially useful as an air intake muffler for an engine;

FIG. 8 is a partial sectional side view of another embodiment of the invention wherein the elastic valving band has a largely flat cross section;

FIG. 9 is a partial sectional side view of a muffler constructed in accordance with another embodiment of the invention, in which the walls of the exhaust port extend radially into the chamber of the muffler;

FIG. 10 is a partial sectional side view of another embodiment of the invention, wherein the valving band includes both metal and elastomeric portions;

FIG. 11 is a side view of another embodiment of the invention, wherein the valving band lies within the muffling chamber;

FIG. 12 is a view of area 12-12 of FIG. 11;

FIG. 13 is a sectional front view of a muffling device, which indludes a valving band containing a flexible, but not necessarily elastic, portion and a separate spring portion; and

FIG. 14 is a sectional side view of a muffler in which the valving bands lie within an inner shell that is surround by an outer shell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 illustrate a muffler 10 which is designed for mounting on the exhaust tube of a motorcycle engine or the like to greatly reduce noise from the engine. The muffler includes a shell 12 which forms a chamber 14 with an entrance opening 16 through which exhaust gases are received from the engine. The shell has a large cylindrical portion 18 with a series of grooves 20 formed therein, and with a group of holes 22 spaced along each of the grooves. A series of elastic bands 24 lies in the grooves over the holes therein to cover the holes.

Exhaust gases that enter the chamber through the entrance opening 16 could readily exit through the exhaust holes or ports 22, except that the ports are sealed against the free outflow of gases by the elastic bands 24. The bands 24 are maintained in a preloaded or stretched state, so that it requires a substantial force to displace them. When the pressure at an exhuast port 22 rises above a certain level, the pressure is sufficient to deflect the portion of the band 24 lying over the port, so that gases can exit through the port in the manner shown in FIG. 4. Actually, the portion of the band 24 lying over a hole 22 may deflect sidewardly as to the position 24b, but in any case the deflection of the band away from the exhaust port permits some gas to escape and therefore lower the pressure in the muffler chamber to reduce the back pressure on the engine. Of course, as soon as the localized pressure falls, the portion of the elastic band 24 which was deflected returns to its original position wherein it seals the port 22 to the outflow of gas and sound.

The muffler can greatly reduce the amount of sound emanating from an engine without producing a high back pressure that would seriously decrease engine efficiency. It is believed that the high effectiveness of the muffler is due to the fact that moving high pressure areas generated by the exhaust do not coincide with shock or sound waves produced by the engine exhaust. The exhaust ports 22 are closed most of the time, so that the sound waves normally cannot pass through them. The small intensity and very brief pressure increase represented by the sound waves is not sufficient to deflect a band portion away from a hole. However, the exhaust ports become open during the passage thereby of a pressure wave caused by the movement of gases. Thus, exhuast gases are allowed to escape at times when the pressure is high enough to permit a rapid escape. The bands 24 therefore serve as valve means which keep each exhaust port closed except at times when a predetermined local pressure is present at the port. When such a pressure is present, the band valve means opens the exhaust port, but only so long as the high pressure continues and it then closes the exhaust port so that any sound waves which may arrive soon afterwards cannot pass to the outside of the muffler.

The shell 12 of the muffler may be formed from steel or other material which can withstand a high temperature. The front end near the entrance opening 16 is constructed of a smaller diameter than the perforated cylindrical portion at 18 to facilitate connection to an exhaust pipe, and the front end is constructed with thicker walls to strengthen it. The rearward end of 10E contains a steel plate 26 to close it, and a plate 28 of asbestos or other high heat-resisting material is placed in front of the steel plate to receive the pulses of hot exhaust gases which are directed thereat through the entrance opening 16. The elastic bands 24 may be constructed of an elastomeric material such as a silicon rubber which can withstand high temperatures. The fact that the bands lie on the outside of the shell results in cooling by the ambient atmosphere.

The muffler 10 with the closed rearward end 10E opposite the entrance 16, is especially useful on a twocycle engine. FIG. 5 shows a two-cycle engine arrangement which includes a cylinder 40 that surrounds a piston 42 which is shown at the bottom of a stroke. As the cylinder moves down towards the position of FIG. 5, it uncovers an exhaust opening 44 and an entrance opening 46, so that burned gases can flow out through the exhaust opening 44 while a fresh charge of air and fuel can enter through the opening 46. The exhaust opening 44 is connected to a conventional expansion chamber 48 which includes an expansion portion 50 and a contraction portion 52. The expansion portion 50 generates a reflected negative pressure at the engine exhaust opening 44 to aid in the exhaust of burned gases from the exit opening 44 so that a fresh charge can enter the cylinder 40 of the engine. The contraction portion 52 serves to generate a positive reflected pressure wave which is reflected back towards the exhaust opening 44 of the engine. This reflected positive pressure wave preferably arrives at the opening 44 after most of the burned gases have been exhausted, and serves to resist the outflow of the fresh charge that has entered the cylinder. The reflected positive pressure wave serves to partially close the exhaust port 44 until the piston 42 rises high enough to seal the port 44.

The expansion chamber 48 for a two-cycle engine is normally designed so that the wave reflected from the contraction portion 52 is most effective at high speed operation of the engine. At low speeds, the reflected waves will reach the exhaust opening 44 sooner than is desirable, and some of the fresh charge will leak out the opening 44 before the piston can close it. The muffler 10 of the present invention, however, serves to provide a second positive reflected wave to maintain good efficiency even at low engine speeds. The rear end wall 10E of the muffler serves to provide a positive reflected wave which returns to the exhaust opening 44 of the engine at a time after the wave reflected from the contraction portion 52 arrives thereat. This second reflected wave is especially useful at low engine speeds to resist the outflow of the fresh charge from the exhaust opening 44 until the piston 42 can close it. In mufflers commonly used heretofore for two-cycle engines, a second reflected wave of substantial intensity was not available because shock waves would be dissipated in the muffler together with the pressure of the exhaust. The muffler of the present invention, however, by retaining the acoustical shock wave, can provide a large second positive reflected wave without producing a high back pressure at the muffler. Tests which have been conducted on two cycle engines using mufflers of the type illustrated in FIGS. 1-5 have been found to greatly increase the efficiency of the engine at low engine speeds and have also been found to provide an ap preciable though smaller increase in efficiency even at high engine speeds as compared to the efficiency obtained with presently available conventional mufflers. Also, the muffler of the present invention eliminates the pulsating stream of hot gases issuing from a conventional muffler, and instead generates only a diffuse foglike emission. Furthermore, the presence of only small exhaust ports which are only slightly opened results in the muffler serving as a spark arrester.

FIG. 6 illustrates a muffler 60 similar to the muffler 10, but with a plurality of fins 62 extending from the shell of the muffler between the band-receiving grooves. The fins 62 serve not only to cool the muffler, but also serve to prevent loss of the bands 24. The bands 24 otherwise can be readily removed from the muffler and could be easily removed by vandals.

FIG. 7 illustrates a muffler which is designed as an air intake muffling device for an engine. In this muffler, air enters through an opening 72 in an inner shell 74, and exits through an exit opening 76 in an outer shell 78 that surrounds the inner shell 74. The inner shell 74 is cylindrical and has grooves with holes 80 through which air moves, and has bands 82 similar to the bands 24 for sealingly covering the holes 80. The intake opening of an engine generates repeated vacuum or negative pressure pulses as the intake is repeatedly opened. These pulses generate noise, although the noise level is generally below the level generated at the exhaust. However, even this noise at the intake can be reduced by the muffler 70 wherein each of the ports 80 through which air can flow is closed except when the localized pressure outside a band 82 drops to a low level. Then, the band 82 deflects to allow air to pass through a port 80 for a brief time. The band portion quickly closes over the port as soon as the negative pressure pulse no longer exists at the port, so that sound waves cannot pass through the port. The travelling negative pressure regions and sound waves do not necessarily coincide, so that the brief opening of the ports does not allow a large proportion of the sounds to escape. In the intake filter 70, the gaseous medium passing through the muffling device moves in a direction opposite to noise, instead of in the same direction as occurs in the case of an exhaust muffler. However, the mufflers basically operate in a similar manner.

The muffler devices can be constructed with a wide variety of designs of elastic bands, grooves for receiving the bands, and arrangements of ports to be covered by the bands. In the muffler described hereinabove and best illustrated in FIG. 3, the groove 20 is of semicircular cross section and the band 24 is of circular cross section to closely mate with the groove. FIG. 8 illustrates a portion of a muffler having a band 90 whose cross section defines a central portion 92 that lies over a pair of holes 92, 94, in the shell 96 of the muffler, the elastic band also having a pair of side portions 98, 100 on either side of the central portion 92. The side portions 98, 100 serve as flaps that are biased against the shell 96 but which can be locally deflected away from the shell 96 to permit the escape of the gaseous or other medium.

FIG. 9 illustrates a muffler similar to that of FIGS. 1-5 except that the shell 110 of the muffler forms a protruding portion 112 that protrudes into the chamber 114 of the muffler, with the side walls 116, 118 of the protrusion extending substantially perpendicular to the surrounding wall portions of the shell 110. This arrangement produces an especially low noise level. It is believed that the protruding portion 112 about the exhaust port 120 serves to deflect sound waves away from the ports 120, so that less sound passes therethrough. Also, the greater length of the resulting ports 120, which are longer than the thickness of most of the shell, serves to attenuate acoustic waves.

FIG. illustrates another arrangement wherein a band 130 is utilized which is an assembly of a resilient core wire 132 that is embedded in an elastic strip 134. The core wire 132 may be constructed of steel or other material while the band portion 134 may be constructed of rubber. A pair of exhaust ports 136, 138 lie under the band 130 and are sealed by flaps 134a and 134b of the band portion 134 that lie on either side of the core wire 132. The pressure of gases in the ports 136, 138 can cause the flaps 134a, l34b to deflect away from the shell 140 to permit the escape of gases through the exhaust port.

FIGS. 11 and 12 illustrate another muffling device, especially useful as an intake muffler, in which the valving means includes bands that are located within the shell 152 of the muffler instead of outside of it. The shell has internal grooves 154 with ports 156, 158 arranged in the grooves, and the valving bands 150 lie in the grooves. An expander member or ring 160, which lies within each band, and which is constructed of resilient material such as spring-tempered steel, tends to expand to a diameter greater than the diameter of the groove 154 in the shell, so that the expander ring urges the band against the walls of the grooves to cover the ports 156, 158. The bands 150 are constructed of flexible material and preferably elastomeric material, and extend in a closed loop or ring to fit the grooves 154 that extend in circles around the shell. Each band has a middle portion 161 lying at the bottom of the groove and has a pair of flap portions 162, 164. The flap portions 162, 164 lie on either side of the expander ring 160, and are biased towards a closed position over the ports. A localized pressure differential at the ports I56, 158, which urges the inrush of air through the ports, causes local regions of the flap portions 162, 164 to deflect inwardly and allow the passage of air therethrough as indicated by arrows 165. Further muffling is provided by a thick layer or blanket 166 of porous elastomeric material such as foam rubber or a resilient foamed plastic, that lies over the grooves 154. The blanket 166 also serves as a filter, so that when the front end 168 of the intake muffler, which has an exit opening 170, is connected to the intake of an engine, the muffler also serves as an air filter.

FIG. 13 illustrates still another arrangement in which a band of flexible, but not necessarily resilient material, is utilized which has ends 180E that are spaced apart and that are biased together by a separate spring 182. The spring 182 holds the band 180 against the walls of the muffler chamber 184 and tightly over holes 186 in the walls. The pressure of a medium within the chamber 184 causes deflection of the band away from the holes to allow the escape of the medium within the chamber. The force of the band 180 against the chamber 184 varies around the circumference of the chamber, with the force being highest at the location 188 op posite the spring 182. In order to compensate for this varying force, the size of the holes 186 is varied around the circumference of the chamber, with the holes being largest at the region 188 where the force is highest. The arrangement of FIG. 12 permits a wide variety of materials to be utilized for sealing the holes 186. For example, the band 180 may be a steel wire which might not have sufficient resiliency in lengthwise stretching to be used without a separate spring of the type shown at 182.

FIG. 14 illustrates a muffler 200, designed for connection to the exhaust of an engine, which is especially quiet, clean, and vandal-resistant. The muffler includes an outer shell 202 with an entrance opening 204 for receiving exhaust gases, and an inner shell 206 disposed within the outer shell and having an exit opening 208 through which exhaust gases are emitted. The inner shell 206 has internal grooves 210 with ports 212 therealong. Internal valving means 214 that include V- shaped bands 216 and expander rings 218 similar to those of FIGS. 11 and 12, lie in the grooves to control the flow of exhaust gases into the inner shell. A group of four air tubes 220 directly couple the front of the inner shell 206 to the ambient atmosphere outside the outer tube, to allow cooling air to be drawn into the inner tube. A pair of perforated discs 222, 224 near the exit opening help to block the escape of noise, while a nozzle-like expanding exit opening 208 helps to reduce the speed of the escaping exhaust to also reduce noise.

The muffler of FIG. 14 provides a clean appearance because it prevents the accumulation of oil on the outside thereof, except for oil that may accumulate at the exit opening 208. Engines, and particularly two-cycle engines, often emit unburned oil in their exhaust. This oil tends to build up at the valving bands in mufflers of this invention. Where the valving bands are on the outside of the muffler, the oil on the bands can create a dirty apperance and can soil clothing that brushes against the side of the muffler. In the muffler 200, the oil may eventually reach the exit opening 208, but this is a more localized area. The fact that the bands 216 are within the muffler also tends to prevent their re moval by vandals. The bands could be placed on the outside of the inner shell, as in the muffler of FIG. 7, but this region is hot, which could result in a shorter band life. By placing the bands inside the inner shell 206, the bands are situated in a cooler environment. It may be noted that much of the heat of the exhaust gases is dissipated through the walls of the outer shell 202 so that the gases are cooler when they pass by the valving bands. Gases entering the outer tube can collect first near the rear end 202R of the outer tube where there is an enlarged area to receive them. The region inside the inner shell 206 is cooled by the air tubes 220, which take in ambient air to mix with exhaust gases to keep the valving bands cooler. The use of an inner shell 206 within an outer one results in especially good noise muffling because the reduced noise passing by the valving bands is further reduced in passing out of the inner shell.

The bands or other valving members which keep the numerous ports of a muffler closed until the localized pressure exceeds a predetermined level, should cover the ports in a sealing arrangement therewith. That is, the ports should be substantially completely closed. A band constructed of spring wire would in a tight helix ordinarily cannot be used in place of the elastic band 24, for example, because the space between the turns of the helix would leave a gap over the ports of the muffler through which sound could escape, especially if common wire of round cross-section is used. When all of the ports are well sealed by the bands or other valving means, the noise insulation can be very great. A muffler of the form illustrated in FIGS. 1-4 has been constructed and tested on a motorcycle engine, with a shell 12 of approximately 3 inches diameter and with the holes 22 being of approximately Vs inch diameter. The total opening area existing when all bands were away from their holes was 2.69 square inches, so that a considerable area was available to provide effective gas exhaust. With all twelve bands 24 securely in place, only a small level of noise was heard to emerge from the muffler. Yet when a portion of one band 24 was held away from the holes, so that only about 1% of the port area was open, the noise level was heard to greatly increase. The elastomeric bands, such as those of silicon rubber, are believed to further decrease the noise level by reason of their dampening effect on the shell of the muffler which minimizes muffler vibration. Also, as a band is deflected and snaps back against the muffler shell, only a minimum of noise is produced because of the use of elastomeric material. Each of the bands 24 is a separate closed loop. While a continuous helix of elastomeric material could be utilized which would be anchored only at its ends, to cover holes arranged along a corresponding helix, a series of separate bands is generally preferable because it assures a more uniform or controlled band tension. It should be noted that it is not necessary for all of the bands to have the same tension or preloading even if all of the ports are of the same size, inasmuch as the pressure distribution can vary along the length of the muffler and the band tensions can be varied accordingly. It also should be noted that the ports covered by the bands, do not have to be round holes, but may be elongated slots or have any of a variety of shapes. The shell itself, in which the ports are formed, may be of circular cross-section, as illustrated in the drawings, but may instead be oval so as to have a smaller width, which is important in motorcycles where the width of the vehicle is limited. Additional elements can be added to further decrease the noise level, although this makes the muffler more complicated. For example, in a muffler of the type illustrated in FIGS. l-4, a louvered tube of the diameter of the entrance opening 16 and having a length slightly less than that of the entire muffler, was installed within the shell 12 to reduce the sound before it reached the bands 20, and this was found to produce an additional noise reduction.

Thus, the invention provides a rnuffling device which is especially useful under circumstances where pressure waves are generated, to pass a medium while minimizing the passage of noise or acoustic waves. The medium may be a variety of fluids including gases from an engine exhaust or hydraulic liquid in which acoustic waves, or pressure disturbances, are being propagated. Particularly in the case of a gaseous medium such as that from an engine exhaust, where only relatively low pressures are encountered, bands of elastomeric material can be readily utilized to seal the ports through which the medium passes. The muffler of the invention is especially useful for attachment to the exhaust of a two cycle engine. A muffler for such an engine may be constructed with a closed end opposite the entrance through which exhaust gases are received, to generate a reflection wave which helps to close the exhaust port of the engine to thereby increase the efficiency of the engine.

Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An exhaust muffler comprising: an outer shell having a front end with an entrance opening and having a rear end which is closed;

an inner shell with at least a front portion disposed within said outer shell, said inner shell having a rear end with an exit opening, said inner shell having a plurality of ports; and

valving band means disposed within said inner shell against said ports, said band means tending to expand so it seals the ports closed except when the localized pressure differential at each port is great enough to deflect a portion of the band means away from the port.

2. The muffler described in claim I wherein:

said inner shell has a plurality of internal grooves,

said ports being disposed along said grooves; and

said valving means includes a plurality of elastomeric bands disposed in said grooves and a plurality of resilient expander rings lying within said bands to urge the bands outwardly against the walls of the grooves.

3. The muffler described in claim 1 including:

at least one air tube having an inner end connected to the inner shell at a location forward of said band means and an outer end open to the ambient atmosphere, for drawing in air to mix with exhaust gases to thereby help cool the band means.

4. A muffler comprising:

walls forming a chamber with an opening for passing a medium, said chamber having a substantially cylindrical portion forming a multiplicity of ports; and

a plurality of valve means for sealingly covering said ports to prevent the passage of acoustic waves therethrough, said valve means including an elastic band device lying within said substantially cylindrical chamber portion at said ports, said band device tending to expand to a diameter larger than that of the cylindrical chamber portion so that it is biased against the inner wall of the chamber portion but can deflect to open a port when there is a higher pressure on the outside of said wall than at the inside thereof at the location of the port.

5. The combination of an engine and exhaust muffler comprising:

an internal combustion engine including an engine chamber, means for introducing fuel and air into said engine chamber and igniting them to produce a rapidly increased gas pressure, piston means movable in said chamber by said gas pressure for producing work, and an outlet of permitting the exhausting of gas from said chamber;

an exhaust muffler with a wall forming a muffler chamber with an opening for receiving exhaust gas, a groove, and a multiplicity of ports arranged along said groove; and

means for coupling said outlet of said engine to said opening of said muffler;

said muffler including a band of elastomeric material lying in said groove for sealingly covering said ports to prevent the passage of acoustic waves therethrough, said band opening said ports when there is a pressure differential of the magnitude produced by said engine in normal operation between opposite sides of the wall at the location of the ports, whereby shock waves resulting from the fuel ignition in the engine and which travel through the exhaust gases, are hindered from escaping into the ambient atmosphere with the exhaust gases.

6. The combination described in claim 5 wherein:

said muffler chamber includes an elongated substantially cylindrical portion, with said ports lying in at least four groups, each group of ports spaced along a substantially circular path on said cylindrical muffler chamber portion and said circular paths being spaced along the length of said elongated chamber portion; and

said muffler includes at least four bands of elastomeric material, each extending in a closed loop about one of said substantially circular paths;

the distance of the bands which are furthest apart being greater than the radius of said bands.

7. The combination describes in claim 5 wherein:

said band lies inside said muffler chamber and tends to expand against the wall of said chamber against said ports. 

1. An exhaust muffler comprising: an outer shell having a front end with an entrance opening and having a rear end which is closed; an inner shell with at least a front portion disposed within said outer shell, said inner shell having a rear end with an exit opening, said inner shell having a plurality of ports; and valving band means disposed within said inner shell against said ports, said band means tending to expand so it seals the ports closed except when the localized pressure differential at each port is great enough to deflect a portion of the band means away from the port.
 2. The muffler described in claim 1 wherein: said inner shell has a plurality of internal grooves, said ports being disposed along said grooves; and said valving means includes a plurality of elastomeric bands disposed in said grooves and a plurality of resilient expander rings lying within said bands to urge the bands outwardly against the walls of the grooves.
 3. The muffler described in claim 1 including: at least one air tube having an inner end connected to the inner shell at a location forward of said band means and an outer end open to the ambient atmosphere, for drawing in air to mix with exhaust gases to thereby help cool the band means.
 4. A muffler comprising: walls forming a chamber with an opening for passing a medium, said chamber having a substantially cylindrical portion forming a multiplicity of ports; and a plurality of valve means for sealingly covering said ports to prevent the passage of acoustic waves therethrough, said valve means including an elastic band device lying within said substantially cylindrical chamber portion at said ports, said band device tending to expand to a diameter larger than that of the cylindrical chamber portion so that it is biased against the inner wall of the chamber portion but can deflect to open a port when there is a higher pressure on the outside of said wall than at the inside thereof at the location of the port.
 5. The combination of an engine and exhaust muffler comprising: an internal combustion engine including an engine chamber, means for introducing fuel and air into said engine chamber and igniting them to produce a rapidly increased gas pressure, piston means movable in said chamber by said gas pressure for producing work, and an outlet of permitting the exhausting of gas from said chamber; an exhaust muffler with a wall forming a muffler chamber with an opening for receiving exhaust gas, a groove, and a multiplicity of ports arranged along said groove; and means for coupling said outlet of said engine to said opening of said muffler; said muffler including a band of elastomeric material lying in said groove for sealingly covering said ports to prevent the passage of acoustic waves therethrough, said band opening said ports when there is a pressure differential of the magnitude produced by said engine in normal operation between opposite sides of the wall at the location of the ports, whereby shock waves resulting from the fuel ignition in the engine and which travel through the exhaust gases, are hindered from escaping into the ambient atmosphere with the exhaust gases.
 6. The combination described in claim 5 wherein: said muffler chamber includes an elongated substantially cylindrical portion, with said ports lying in at least four groups, each group of ports spaced along a substantially circular path on Said cylindrical muffler chamber portion and said circular paths being spaced along the length of said elongated chamber portion; and said muffler includes at least four bands of elastomeric material, each extending in a closed loop about one of said substantially circular paths; the distance of the bands which are furthest apart being greater than the radius of said bands.
 7. The combination describes in claim 5 wherein: said band lies inside said muffler chamber and tends to expand against the wall of said chamber against said ports. 